This section provides background information related to the present disclosure which is not necessarily prior art.
Some cardiac surgery procedures require peripheral artery cannulation for cardiopulmonary bypass. Also, some disease states require mechanical cardio-pulmonary support via peripheral artery cannulation. This peripheral artery is often, but not always, the femoral artery. Insertion into the femoral artery of an arterial cannula of sufficient size to support the patient with cardiopulmonary bypass often leads to compromised blood flow to the lower limb which can lead to ischemia and tissue necrosis during prolonged procedures.
Previously proposed methods of providing perfusion to the body while maintaining perfusion to the lower limb are generally inconvenient and often do not provide a satisfactory solution.
It has previously been proposed to use an undersized cannula, based on the assumption that the smaller cannula will allow blood flow back over the body of the cannula between the cannula body and the arterial wall. In practice it is difficult to achieve adequate perfusion to the lower limb and using a cannula of a smaller size than what is actually required compromises perfusion to the body and increases line pressures, thus increasing the risk of red blood cell haemolysis, increased backpressure to the membrane oxygenator and the perfusion pump and the increased risk of damage to these vital pieces of equipment.
It has also been previously proposed to insert a further perfusion cannula downstream of a first, main perfusion cannula. Inserting a downstream cannula can be technically difficult and a percutaneous approach often requires ultrasound guidance to allow accurate placement. This technique requires an extra cannula and an extra perfusion line that must be connected into the arterial side of the perfusion circuit and can be time consuming. It also results in extra hardware being disposed in the groin incision area, an area that is already compromised for space with the femoral arterial and femoral venous lines already in place. The downstream cannula would typically be a small cannula which is more susceptible to positional changes, resulting in less reliable downstream flow.
It has also been previously proposed to sew a side graft to the artery when using femoral artery cannulation. In this technique, surgeons sew on a Dacron graft to the side of the femoral artery as an end to side anastamosis and a cannula is inserted into the graft. This technique is time consuming, taking approximately 30 minutes to sew on the graft and cannulate it, compared with approximately 2 minutes to insert a bidirectional femoral cannula. Furthermore, this technique requires an open surgical procedure which can be difficult in an ICU setting. Bleeding can also be a problem in patients requiring extended periods of support and when support ceases, the base of the Dacron graft may be left in situ with this technique, creating a potential source for thrombus formation and infection.
It is desirable to provide a single cannula that provides adequate perfusion to the lower limb. However, as will be discussed below, previously proposed cannulae suffer a number of drawbacks.
It has been previously proposed to provide a conventional cannula with side perfusion holes through which blood can flow toward the lower limb. Such arrangements have been disclosed in WO03/068303 to Laksen et al. and in “A Novel Femoral Arterial Cannula to Prevent Limb Ischemia During Cardiopulmonary Support Preliminary Report of Experimental and Clinical Experiences” by Matsui et al. in Artif Organs, Vol. 30, No. 7 2006. In arrangements having side perfusion holes, the cannula must be correctly positioned in the artery so that the holes are not occluded, and maintained in that position. In these arrangements, no tactile feedback is available to assist with positioning of the holes and assistance in maintaining the holes in correct position is not provided. If the cannula migrates distally the holes will be occluded by wall of artery. If the cannula migrates proximally then the holes may move outside of the artery and cause bleeding. If the side holes are at level of the arteriotomy, perfusion into the wall of the artery may cause a dissection.
To prevent occlusion of side holes provided in a conventional cannula, it has been proposed to provide rails adjacent the holes to prevent occlusion of the holes. Such an arrangement has been disclosed in “A femoral artery cannula that allows distal blood flow” by Magovem, J. et al. (The Journal of Thoracic and Cardiovascular Surgery, September 2005). The configuration of the rails can be complicated and difficulties will be encountered with their insertion and removal through the wall of the artery. The rails also create a ridged cross section which creates a potential for bleeding during insertion and removal. Furthermore, blood passing through the side holes is not efficiently communicated as it is directed against the wall of artery.
Alternative rail configurations have also been proposed to facilitate insertion of the cannula into the artery while attempting to prevent occlusion of side holes. Documents U.S. Pat. No. 5,171,218 and U.S. Pat. No. 5,330,433 by Fonger, J. et al, each disclose an arrangement in which the rails are in the form of forward pointing barbs between which a slanted elongate hole located in a depression on the exterior of the wall of the cannula is disposed. The depression impinges into the main lumen to act as a scoop to divert blood toward the lower limb.
As with previous proposals, difficulties will be encountered with insertion and removal due to the cross sectional shape of the barbs, which, as can be seen in FIG. 5 of each of the documents, is ridged in the region of the barbs/rails and will require the artery to stretch during insertion and removal. As this region passes through the artery wall, this ridged cross section may also create channels between the ridges that may result in bleeding during insertion and removal.
Furthermore, the main lumen is narrowed by the side hole depression, thereby reducing flow capability. The most important determinant of flow through a cannula, as determined by the Poiseulle-Hagen equation, is cannula radius. Decreasing radius by half causes a decrease in flow of sixteen times. Reducing the radius in a femoral cannula that is already being pushed to achieve maximal flow rates is a major compromise in the primary function, that is, providing flow equal to systemic cardiac output.
Due to the configuration of the barbs and the side aperture, this arrangement may be difficult to manufacture. Furthermore, flow from the side aperture does not have an open area of artery to flow into, thereby reducing flow efficiency and creating an area of turbulent flow.
As discussed above, each of the previously proposed bi-directional cannulae have suffered from a number of problems. Furthermore, general poor performance has been observed due to issues with occlusion of side facing holes/apertures. The Inventors have found that the previous poor performance can at least partially be attributed to two factors, arterial spasm and downstream compression.
Arterial Spasm relates to the normal physiological response of contraction of arterial smooth muscle to stretch or local trauma. Arterial spasm around the body of the cannula will result in a reduction of blood flow back around the cannula and down the leg. This can even occur around undersized cannula.
Downstream Compression, as illustrated in FIG. 1, is a mechanism that has not been previously acknowledged. The body of a standard femoral cannula causes a distortion of the arterial wall around the point of insertion. As the cannula tends to lie in the orientation of the artery, the body of the cannula causes a downward displacement of the distal edge of the arteriotomy, and compression of the artery just distal to the arteriotomy. Obstruction of flow downstream to the arteriotomy has to be overcome if reliable downstream flow is to be provided.
The issues of arterial spasm and downstream compression have not been acknowledged or addressed in the previous proposals.
Examples of the invention seek to solve, or at least ameliorate, one or more disadvantages of previous cannulae.